3rd member swap advice

Discussion in 'Transmission and Drivetrain Tech' started by demon322, Apr 20, 2017.

  1. demon322

    demon322 FABO Gold Member FABO Gold Member

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    Trying to plan out my summer projects.

    I want to swap in this new 3rd member to replace my open (I think)3.23s and gain sure grip.

    489 case with 3.91s

    This was pulled from my fathers car several years back and has just been sitting since. I did nothing but clean it up and paint over the surface rust. Is there anything I should check or do before the swap? Best I can gather from him it was working fine before he removed it he just wanted a more highway friendly gear. He says it might have had a slight growl but it's been a bit and he's not positive.

    Ideally I can just slap it in there for now as I may be changing gears soon anyway. Afraid the 3.91s may be a bit too low for my liking but hoping my 28in tire will offset that some.

    Sorry for the long post. Any tips or advice appreciated.

    IMG_4830.JPG

    IMG_4831.JPG
     
  2. Richard McLaughlin

    Richard McLaughlin 1974 Duster 340

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    This is exactly what I planing this month as well. I have 3.23 open dif, I have located a 3.91 sure grip to replace that.

    I am sure the 3.91 may be a bit harsh on the highway but I think I am handle that not prob. Especially with all the street advantages it offeres.

    I can't wait to feel the "seat of the pants" the 3.91 offeres.
     
  3. younggun2.0

    younggun2.0 Well-Known Member

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    You will love it! Unless of coarse you sit down to pee!
     
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    • harrisonm

      harrisonm Well-Known Member

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      Third members are pretty easy to put in. I'd just clean it up really well, inspect it closely for bad gears or growling bearing and put it in. Use a good quality synthetic gear lube.
       
    • Dubob

      Dubob Well-Known Member

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      Make sure the "buttons" are still in the center where the axles go in for the lash adjustment.
       
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      • younggun2.0

        younggun2.0 Well-Known Member

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        Unless using green bearings
         
      • rustycowll69

        rustycowll69 Well-Known Member

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        you could check the mesh pattern.
         
      • jbc426

        jbc426 Well-Known Member

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        This !

        I also take this opportunity to clean out the housing. I use that purple cleaner degreaser, a brush and the garden hose. Once clean, I blow out the water with the gas leaf blower and use some rags to get the rest.

        It's a good time to replace seeping axle seals and freshen the brakes too.
         
      • Richard McLaughlin

        Richard McLaughlin 1974 Duster 340

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        What kind of paint did you use? Is it a good idea to do that? Any benifit? I'm picking up mine in a few weeks so I would do the same if it is recommended.
         
      • Richard McLaughlin

        Richard McLaughlin 1974 Duster 340

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        Do you take it apart to do this? Like take the gears out etc?
         
      • jbc426

        jbc426 Well-Known Member

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        I just clean the housing and brake backing plates etc. The 3rd member should be clean when pulled and stored. If it's not, I use some brake cleaner to rinse any grit and foreign matter out of it before putting it together. Changing the pinion seal if it is seeping is easy to do as well if you have an air tool to zip the nut off. Don't get water on the 3rd member internals.

        The rear end will grind most contaminants to dust in a few hundred miles, but its best if there are none to begin with. If you change the gear oil after 1000 miles when warm, you will flush a lot of the any remaining contamination out. I use Dinosaur oil for the first fill and synthetic for the long run. Be sure to add the anti chatter additive if you have a clutch type diff versus the cone type.
         
      • twayne24365

        twayne24365 Well-Known Member

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        make sure the 3rd members have the same yoke/u joint size
         
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        • demon322

          demon322 FABO Gold Member FABO Gold Member

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          This is what I was hoping. Not that difficult to pull back out if something major is wrong. Gears look to be in good shape. How do you check to see if there is bearing noise? Thanks.
           
        • demon322

          demon322 FABO Gold Member FABO Gold Member

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          I'm not sure what you mean by this. Is there something I should look for?
           
        • demon322

          demon322 FABO Gold Member FABO Gold Member

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          Green wheel bearings? I bought new axle shafts from Dr Diff and they have green bearings.
           
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          • demon322

            demon322 FABO Gold Member FABO Gold Member

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            Is this hard to do? I do have the fsm for 71 - is it explained there? Thanks.
             
          • demon322

            demon322 FABO Gold Member FABO Gold Member

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            No benefit just thought it looked a little better. Mine is John Deere blitz black. Hell, the whole car is at the moment ha.
             
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            • demon322

              demon322 FABO Gold Member FABO Gold Member

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              It was fairly clean when it was pulled. Good idea about the pinion seal though since it is easy to get to now. Thanks.
               
            • Bad Sport

              Bad Sport HALF A BUBBLE OFF Staff Member FABO Gold Member

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              In all likely hood, the thrust buttons are still in the suregrip. You are lucky and have the clutch type so they can be removed easily. Look through the axle splines where the axles go in and see if the buttons are in there.

              If so, they are simply held together with a pin that is flared on the ends. They need to be knocked out.
               
            • demon322

              demon322 FABO Gold Member FABO Gold Member

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              Also a good tip. If they are not do you reccomend swapping yokes or just using the u-joint with the 2 different sizes? Thanks.
               
            • demon322

              demon322 FABO Gold Member FABO Gold Member

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              Thanks! I will check this and report back.
               
            • Bad Sport

              Bad Sport HALF A BUBBLE OFF Staff Member FABO Gold Member

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              I just looked at the top pic, it looks like a hole going straight through.

              IMG_4830.JPG
               
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              • rustycowll69

                rustycowll69 Well-Known Member

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                it's probably just my paranoia speaking, but the bottom cap bolt in the last pic looks like it's not tightened all the way, because the parting line looks too wide. It could just be an optical illusion, maybe check it with a torque wrench, or check it with some really small feeler gauges.
                 
              • twayne24365

                twayne24365 Well-Known Member

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                if the u joints are not the same, the conversion u joint will be fine unless your making some power....just make sure you get the right one for your yoke,
                inside clips or outside locating tabs
                 
              • d55dave

                d55dave Well-Known Member

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                If you have stock adjustable tapered wheel bearings on your axles you need to have the thrust plug (button) installed in the middle of the diff. If you have aftermarket sealed ball type axle bearings (commonly referred to as green bearings, non adjustable) you do not. There is much debate on whether greens are OK to run on the street. My personal opinion is that factory style tapered rollers are superior. If you have tapered rollers you will need to read up on axle adjustments.

                If the u joint is a different size you will need to order a "conversion" joint. They are commonly available. Do not intermix the u joint retaining straps between the small (7260) and large (7290) joints. They appear very similar and can be confused but are dimensionally different.

                Checking backlash and pattern can be done now, however if the used gearset has been run for any length of time with a bad pattern and backlash it will be worn and will always be noisy even if you try to correct it. If the backlash is ballpark you should just leave the adjustments alone. Do check that the pinion nut is tight.

                This is a prime time to check the sure grip clutches. They could be worn out and it may act as an open differential. They can be replaced. Also prime time to service the rear brakes and axle seals.

                Your speedo will read all wrong with the new gears. If it bothers you the drive gear in the transmission will need to be changed.

                There is also some debate on if you need to run a gasket between the third member and housing. Again my opinion is that it should be there as it spaces the third member slightly forward for proper axle alignment.

                All of the above procedures are in the FSM. Dr. Diff is a good source of parts and info.

                Good luck and have fun. The difference with 3:91s will blow your mind.
                 
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